Logic control for fast-acting safety system

ABSTRACT

Woodworking machines including cutting tools and motors adapted to drive the cutting tools are disclosed. The machines also include a detection system adapted to detect a dangerous condition between the cutting tool and a person, and a reaction system adapted to perform a specified action upon detection of the dangerous condition. The machines further include a control system adapted to test the operability of at least a portion of the detection system and/or the reaction system. The control system is adapted to disable the motor if the tested portion is inoperable.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional continuation of U.S. patent applicationSer. No. 09/929,237, which in turn claimed the benefit of and priorityfrom the following U.S. Provisional Patent Applications: Ser. No.60/225,056, filed Aug. 14, 2000, Ser. No. 60/225,057, filed Aug. 14,2000, Ser. No. 60/225,058, filed Aug. 14, 2000, Ser. No. 60/225,059,filed Aug. 14, 2000, Ser. No. 60/225,089, filed Aug. 14, 2000, Ser. No.60/225,094, filed Aug. 14, 2000, Ser. No. 60/225,169, filed Aug. 14,2000, Ser. No. 60/225,170, filed Aug. 14, 2000, Ser. No. 60/225,200,filed Aug. 14, 2000, Ser. No. 60/225,201, filed Aug. 14, 2000, Ser. No.60/225,206, filed Aug. 14, 2000, Ser. No. 60/225,210, filed Aug. 14,2000, Ser. No. 60/225,211, filed Aug. 14, 2000, and Ser. No. 60/225,212,filed Aug. 14, 2000. The complete disclosures of these applications arehereby incorporated by reference in their entireties.

FIELD

The present invention relates to safety systems, and more particularlyto a high-speed safety system for use on power equipment.

BACKGROUND

Beginning with the industrial revolution and continuing to the present,mechanized equipment has allowed workers to produce goods with greaterspeed and less effort than possible with manually-powered tools.Unfortunately, the power and high operating speeds of mechanizedequipment creates a risk for those operating such machinery. Each yearthousands of people are maimed or killed by accidents involving powerequipment.

As might be expected, many systems have been developed to minimize therisk of injury when using power equipment. Probably the most commonsafety feature is a guard that physically blocks an operator from makingcontact with dangerous components of machinery, such as belts, shafts orblades. In many cases, guards are effective to reduce the risk ofinjury, however, there are many instances where the nature of theoperations to be performed precludes using a guard that completelyblocks access to hazardous machine parts.

Various systems have been proposed to prevent accidental injury whereguards cannot effectively be employed. For instance, U.S. Pat. Nos.941,726, 2,978,084, 3,011,610, 3,047,116, 4,195,722 and 4,321,841, thedisclosures of which are incorporated herein by reference, all disclosesafety systems for use with power presses. These systems utilize cablesattached to the wrists of the operator that either pull back a user'shands from the work zone upon operation or prevent operation until theuser's hands are outside the danger zone. U.S. Pat. Nos. 3,953,770,4,075,961, 4,470,046, 4,532,501 and 5,212,621, the disclosures of whichare incorporated herein by reference, disclose radio-frequency safetysystems which utilize radio-frequency signals to detect the presence ofa user's hand in a dangerous area of the machine and thereupon preventor interrupt operation of the machine.

U.S. Pat. Nos. 4,959,909, 5,025,175, 5,122,091, 5,198,702, 5,201,684,5,272,946, and 5,510,685 disclose safety systems for use withmeat-skinning equipment, and are incorporated herein by reference. Thesesystems interrupt or reverse power to the motor, or disengage a clutch,upon contact with a user's hand by any dangerous portion of the machine.Typically, contact between the user and the machine is detected bymonitoring for electrical contact between a fine wire mesh in a gloveworn by the user and some metal component in the dangerous area of themachine. Although such systems are suitable for use with meat skinningmachines, they are relatively slow to stop the motion of the cuttingelement because they rely on the operation of solenoids or must overcomethe inertia of the motor. However, because these systems operate atrelatively low speeds, the blade does not need to be stopped rapidly toprevent serious injury to the user.

U.S. Pat. Nos. 3,785,230 and 4,026,177, the disclosures of which areherein incorporated by reference, disclose a safety system for use oncircular saws to stop the blade when a user's hand approaches the blade.The system uses the blade as an antenna in an electromagnetic proximitydetector to detect the approach of a user's hand prior to actual contactwith the blade. Upon detection of a user's hand, the system engages abrake using a standard solenoid. Unfortunately, such a system is proneto false triggers and is relatively slow acting because of the solenoid.U.S. Pat. No. 4,117,752, which is herein incorporated by reference,discloses a similar braking system for use with a band saw, where thebrake is triggered by actual contact between the user's hand and theblade. However, the system described for detecting blade contact doesnot appear to be functional to accurately and reliably detect contact.Furthermore, the system relies on standard electromagnetic brakesoperating off of line voltage to stop the blade and pulleys of the bandsaw. It is believed that such brakes would take 50 ms-1 s to stop theblade. Therefore, the system is too slow to stop the blade quicklyenough to avoid serious injury.

None of the safety systems mentioned above disclose any method ormechanism for ensuring that the system is operational before setting theblade or other dangerous portion of the machine in motion. In addition,none of the systems mentioned above disclose any method or mechanism forpreventing false triggers during initial startup or for monitoring theoperating status of the machinery to prevent triggering the safetysystem when the blade is stationary. Further, none of theabove-mentioned systems disclose any method or mechanism for allowing auser to disable the safety system under certain conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a machine with a fast-actingsafety system according to the present invention.

FIG. 2 is a schematic diagram of an exemplary safety system in thecontext of a machine having a circular blade.

FIG. 3 is a flowchart diagram of an exemplary self-test logic sequenceaccording to the present invention.

FIGS. 4A-C are flowchart diagrams of an exemplary self-test andoperational sequence according to the present invention.

FIG. 5 is a schematic block diagram of a logic controller according to afirst exemplary implementation of the present invention.

FIG. 6 is a schematic diagram of a user interface according to thepresent invention.

FIG. 7 is a schematic diagram of a firing capacitor charge and testcircuit according to the first exemplary implementation of the presentinvention.

FIG. 8 is a schematic block diagram of a logic controller according to asecond exemplary implementation of the present invention.

FIG. 9 is a schematic diagram of a firing capacitor charge and testcircuit according to the second exemplary implementation of the presentinvention.

FIG. 10 is an isometric view of an exemplary pawl adapted for measuringpawl-to-blade spacing according to the present invention.

FIG. 11 is a schematic diagram of an exemplary circuit for detectingblade-to-pawl spacing according to the present invention.

DETAILED DESCRIPTION

A machine according to the present invention is shown schematically inFIG. 1 and indicated generally at 10. Machine 10 may be any of a varietyof different machines adapted for cutting workpieces, such as wood,including a table saw, miter saw (chop saw), radial arm saw, circularsaw, band saw, jointer, planer, etc. Machine 10 includes an operativestructure 12 having a cutting tool 14 and a motor assembly 16 adapted todrive the cutting tool. Machine 10 also includes a safety system 18configured to minimize the potential of a serious injury to a personusing machine 10. Safety system 18 is adapted to detect the occurrenceof one or more dangerous conditions during use of machine 10. If such adangerous condition is detected, safety system 18 is adapted to engageoperative structure 12 to limit any injury to the user caused by thedangerous condition.

Machine 10 also includes a suitable power source 20 to provide power tooperative structure 12 and safety system 18. Power source 20 may be anexternal power source such as line current, or an internal power sourcesuch as a battery. Alternatively, power source 20 may include acombination of both external and internal power sources. Furthermore,power source 20 may include two or more separate power sources, eachadapted to power different portions of machine 10.

It will be appreciated that operative structure 12 may take any one ofmany different forms, depending on the type of machine 10. For example,operative structure 12 may include a stationary housing configured tosupport motor assembly 16 in driving engagement with cutting tool 14.Alternatively, operative structure 12 may include a movable structureconfigured to carry cutting tool 14 between multiple operatingpositions. As a further alternative, operative structure 12 may includeone or more transport mechanisms adapted to convey a workpiece towardand/or away from cutting tool 14.

Motor assembly 16 includes one or more motors adapted to drive cuttingtool 14. The motors may be either directly or indirectly coupled to thecutting tool, and may also be adapted to drive workpiece transportmechanisms. Cutting tool 14 typically includes one or more blades orother suitable cutting implements that are adapted to cut or removeportions from the workpieces. The particular form of cutting tool 14will vary depending upon the various embodiments of machine 10. Forexample, in table saws, miter saws, circular saws and radial arm saws,cutting tool 14 will typically include one or more circular rotatingblades having a plurality of teeth disposed along the perimetrical edgeof the blade. For a jointer or planer, the cutting tool typicallyincludes a plurality of radially spaced-apart blades. For a band saw,the cutting tool includes an elongate, circuitous tooth-edged band.

Safety system 18 includes a detection subsystem 22, a reaction subsystem24 and a control subsystem 26. Control subsystem 26 may be adapted toreceive inputs from a variety of sources including detection subsystem22, reaction subsystem 24, operative structure 12 and motor assembly 16.The control subsystem may also include one or more sensors adapted tomonitor selected parameters of machine 10. In addition, controlsubsystem 26 typically includes one or more instruments operable by auser to control the machine. The control subsystem is configured tocontrol machine 10 in response to the inputs it receives.

Detection subsystem 22 is configured to detect one or more dangerous, ortriggering, conditions during use of machine 10. For example, thedetection subsystem may be configured to detect that a portion of theuser's body is dangerously close to, or in contact with, a portion ofcutting tool 14. As another example, the detection subsystem may beconfigured to detect the rapid movement of a workpiece due to kickbackby the cutting tool, as is described in U.S. Provisional PatentApplication Ser. No. 60/182,866, the disclosure of which is hereinincorporated by reference. In some embodiments, detection subsystem 22may inform control subsystem 26 of the dangerous condition, which thenactivates reaction subsystem 24. In other embodiments, the detectionsubsystem may be adapted to activate the reaction subsystem directly.

Once activated in response to a dangerous condition, reaction subsystem24 is configured to engage operative structure 12 quickly to preventserious injury to the user. It will be appreciated that the particularaction to be taken by reaction subsystem 24 will vary depending on thetype of machine 10 and/or the dangerous condition that is detected. Forexample, reaction subsystem 24 may be configured to do one or more ofthe following: stop the movement of cutting tool 14, disconnect motorassembly 16 from power source 20, place a barrier between the cuttingtool and the user, or retract the cutting tool from its operatingposition, etc. The reaction subsystem may be configured to take acombination of steps to protect the user from serious injury. Placementof a barrier between the cutting tool and teeth is described in moredetail in U.S. Provisional Patent Application Ser. No. 60/225,206,entitled “Cutting Tool Safety System,” filed Aug. 14, 2000 by SD3, LLC,the disclosure of which is herein incorporated by reference. Retractionof the cutting tool from its operating position is described in moredetail in U.S. Provisional Patent Application Ser. No. 60/225,089,entitled “Retraction System For Use In Power Equipment,” filed Aug. 14,2000 by SD3, LLC, the disclosure of which is herein incorporated byreference.

The configuration of reaction subsystem 24 typically will vary dependingon which action(s) are taken. In the exemplary embodiment depicted inFIG. 1, reaction subsystem 24 is configured to stop the movement ofcutting tool 14 and includes a brake mechanism 28, a biasing mechanism30, a restraining mechanism 32, and a release mechanism 34. Brakemechanism 28 is adapted to engage operative structure 12 under theurging of biasing mechanism 30. During normal operation of machine 10,restraining mechanism 32 holds the brake mechanism out of engagementwith the operative structure. However, upon receipt of an activationsignal by reaction subsystem 24, the brake mechanism is released fromthe restraining mechanism by release mechanism 34, whereupon, the brakemechanism quickly engages at least a portion of the operative structureto bring the cutting tool to a stop.

It will be appreciated by those of skill in the art that the exemplaryembodiment depicted in FIG. 1 and described above may be implemented ina variety of ways depending on the type and configuration of operativestructure 12. Turning attention to FIG. 2, one example of the manypossible implementations of safety system 18 is shown. System 18 isconfigured to engage an operative structure having a cutting tool in theform of a circular blade 40 mounted on a rotating shaft or arbor 42.Blade 40 includes a plurality of cutting teeth (not shown) disposedaround the outer edge of the blade. As described in more detail below,braking mechanism 28 is adapted to engage the teeth of blade 40 and stopthe rotation of the blade. U.S. Provisional Patent Application Ser. No.60/225,210, entitled “Translation Stop For Use In Power Equipment,”filed Aug. 14, 2000 by SD3, LLC, the disclosure of which is hereinincorporated by reference, describes other systems for stopping themovement of the cutting tool. U.S. Provisional Patent Application Ser.No. 60/225,058, entitled “Table Saw With Improved Safety System,” filedAug. 14, 2000 by SD3, LLC, and U.S. Provisional Patent Application Ser.No. 60/225,057, entitled “Miter Saw With Improved Safety System,” filedAug. 14, 2000 by SD3, LLC, the disclosures of which are hereinincorporated by reference, describe safety system 18 in the context ofparticular types of machines 10.

In the exemplary implementation, detection subsystem 22 is adapted todetect the dangerous condition of the user coming into contact withblade 40. The detection subsystem includes a sensor assembly, such ascontact detection plates 44 and 46, capacitively coupled to blade 40 todetect any contact between the user's body and the blade. Typically, theblade, or some larger portion of cutting tool 14 is electricallyisolated from the remainder of machine 10. Alternatively, detectionsubsystem 22 may include a different sensor assembly configured todetect contact in other ways, such as optically, resistively, etc. Inany event, the detection subsystem is adapted to transmit a signal tocontrol subsystem 26 when contact between the user and the blade isdetected. Various exemplary embodiments and implementations of detectionsubsystem 22 are described in more detail in U.S. Provisional PatentApplication Ser. No. 60/225,200, entitled “Contact Detection System ForPower Equipment,” filed Aug. 14, 2000 by SD3, LLC, and U.S. ProvisionalPatent Application Ser. No. 60/225,211, entitled “Apparatus And MethodFor Detecting Dangerous Conditions In Power Equipment,” filed Aug. 14,2000 by SD3, LLC, the disclosures of which are herein incorporated byreference.

Control subsystem 26 includes one or more instruments 48 that areoperable by a user to control the motion of blade 40. Instruments 48 mayinclude start/stop switches, speed controls, direction controls, etc.Control subsystem 26 also includes a logic controller 50 connected toreceive the user's inputs via instruments 48. Logic controller 50 isalso connected to receive a contact detection signal from detectionsubsystem 22. Further, the logic controller may be configured to receiveinputs from other sources (not shown) such as blade motion sensors,workpiece sensors, etc. In any event, the logic controller is configuredto control operative structure 12 in response to the user's inputsthrough instruments 48. However, upon receipt of a contact detectionsignal from detection subsystem 22, the logic controller overrides thecontrol inputs from the user and activates reaction subsystem 24 to stopthe motion of the blade. Various exemplary embodiments andimplementations of logic controller 50 will be described below. Variousexemplary embodiments and implementations of a blade motion detectionsystem are described in U.S. Provisional Patent Application Ser. No.60/225,094, entitled “Motion Detecting System For Use In Safety SystemFor Power Equipment,” filed Aug. 14, 2000 by SD3, LLC, the disclosure ofwhich is herein incorporated by reference.

In the exemplary implementation, brake mechanism 28 includes a pawl 60mounted adjacent the edge of blade 40 and selectively moveable to engageand grip the teeth of the blade. Pawl 60 may be constructed of anysuitable material adapted to engage and stop the blade. As one example,the pawl may be constructed of a relatively high strength thermoplasticmaterial such as polycarbonate, ultrahigh molecular weight polyethylene(UHMW) or Acrylonitrile Butadiene Styrene (ABS), etc., or a metal suchas aluminum, etc. It will be appreciated that the construction of pawl60 will vary depending on the configuration of blade 40. In any event,the pawl is urged into the blade by a biasing mechanism in the form of aspring 66. In the illustrative embodiment shown in FIG. 2, pawl 60 ispivoted into the teeth of blade 40. It should be understood that slidingor rotary movement of pawl 60 may also be used. The spring is adapted tourge pawl 60 into the teeth of the blade with sufficient force to gripthe blade and quickly bring it to a stop.

The pawl is held away from the edge of the blade by a restrainingmechanism in the form of a fusible member 70. The fusible member isconstructed of a suitable material adapted to restrain the pawl againstthe bias of spring 66, and also adapted to melt under a determinedelectrical current density. Examples of suitable materials for fusiblemember 70 include NiChrome wire, stainless steel wire, etc. The fusiblemember is connected between the pawl and a contact mount 72. Preferably,fusible member 70 holds the pawl relatively close to the edge of theblade to reduce the distance the pawl must travel to engage the blade.Positioning the pawl relatively close to the edge of the blade reducesthe time required for the pawl to engage and stop the blade. Typically,the pawl is held approximately 1/32-inch to ¼-inch from the edge of theblade by fusible member 70, however other pawl-to-blade spacings mayalso be used within the scope of the invention.

Pawl 60 is released from its unactuated, or cocked, position to engageblade 40 by a release mechanism in the form of a firing subsystem 76.The firing subsystem is coupled to contact mount 72, and is configuredto melt fusible member 70 by passing a surge of electrical currentthrough the fusible member. Firing subsystem 76 is coupled to logiccontroller 50 and activated by a signal from the logic controller. Whenthe logic controller receives a contact detection signal from detectionsubsystem 22, the logic controller sends an activation signal to firingsubsystem 76, which melts fusible member 70, thereby releasing the pawlto stop the blade. Various exemplary embodiments and implementations ofreaction subsystem 24 are described in more detail in U.S. ProvisionalPatent Application Ser. No. 60/225,056, entitled “Firing Subsystem ForUse In Fast Acting Safety System,” filed Aug. 14, 2000 by SD3, LLC, U.S.Provisional Patent Application Ser. No. 60/225,170, entitled“Spring-Biased Brake Mechanism for Power Equipment,” filed Aug. 14, 2000by SD3, LLC, and U.S. Provisional Patent Application Ser. No.60/225,169, entitled “Brake Mechanism For Power Equipment,” filed Aug.14, 2000 by SD3, LLC, the disclosures of which are herein incorporatedby reference.

It will be appreciated that activation of the brake mechanism willrequire the replacement of one or more portions of safety system 18. Forexample, pawl 60 and fusible member 70 typically must be replaced beforethe safety system is ready to be used again. Thus, it may be desirableto construct one or more portions of safety system 18 in a cartridgethat can be easily replaced. For example, in the exemplaryimplementation depicted in FIG. 2, safety system 18 includes areplaceable cartridge 80 having a housing 82. Pawl 60, spring 66,fusible member 70 and contact mount 72 are all mounted within housing82. Alternatively, other portions of safety system 18 may be mountedwithin the housing. In any event, after the reaction system has beenactivated, the safety system can be reset by replacing cartridge 80. Theportions of safety system 18 not mounted within the cartridge may bereplaced separately or reused as appropriate. Various exemplaryembodiments and implementations of a safety system using a replaceablecartridge are described in more detail in U.S. Provisional PatentApplication Ser. No. 60/225,201, entitled “Replaceable Brake MechanismFor Power Equipment,” filed Aug. 14, 2000 by SD3, LLC, and U.S.Provisional Patent Application Ser. No. 60/225,212, entitled “BrakePositioning System,” filed Aug. 14, 2000 by SD3, LLC, the disclosures ofwhich are herein incorporated by reference.

While one particular implementation of safety system 18 has beendescribed, it will be appreciated that many variations and modificationsare possible within the scope of the invention. Many such variations andmodifications are described in U.S. Provisional Patent Application Ser.Nos. 60/182,866 and 60/157,340, the disclosures of which are hereinincorporated by reference.

Considering logic controller 50 now in more detail, it will beappreciated that the logic controller may be configured to perform avariety of functions depending on the particular type of machine 10and/or the application. For example, logic controller 50 may beconfigured to conduct various self-test safety checks when the machineis switched on or off and during use, to ensure that detection subsystem22 is operating properly and to prevent inadvertent triggering ofreaction subsystem 24. Additionally, the logic controller may beconfigured to control one or more display devices to inform a user ofthe status of machine 10 and safety system 18. Furthermore, logiccontroller 50 may be implemented in a variety of ways including usingone or more custom application specific integrated circuits (ASICs),microprocessors, micro-controllers, digital logic circuits, and/oranalog circuits, etc.

In one exemplary embodiment, logic controller 50 is configured toperform the self-check logic sequence shown in FIG. 3. The exemplarysequence begins when the user initially supplies power to the system,indicated at 901. The logic system first checks to determine whether thespacing between the blade and pawl is correct, as indicated at 902. Theblade-to-pawl spacing may be measured by any suitable mechanism such asdescribed in more detail below. If the spacing is outside acceptablelimits, the system responds with an error signal, indicated at 903. Theerror signal may be an audible and/or visible signal, etc. In oneembodiment described in more detail below, control subsystem includes auser interface adapted to indicate the status of the machine andannunciate any error conditions. Preferably, the logic system remains inthe error state and prevents further operation of the machine until thecorrect blade-to-pawl spacing is detected.

If the blade-to-pawl spacing is acceptable, the logic system determineswhether the input signal produced on charge plate 44 by detectionsubsystem 22 is being detected at a sufficient amplitude on charge plate46, as indicated at 904. This step ensures that the reaction subsystemwill not be triggered accidentally upon start-up due to a fault in thedetection subsystem, a grounded blade, incorrectly placed charge plates,etc. If the proper input signal is not detected, logic controller 50responds with an error signal 903. It will be appreciated that eitherthe same or a different error signal may be produced for each faultcondition.

If the proper input signal is detected, the logic controller proceeds todetermine whether a fusible member is present, as indicated at step 905.The presence of a fusible member may be determined by any suitable meanssuch as described in more detail below. If no fusible member is present,logic controller 50 returns an error signal 903. If a fusible member isdetected, the logic controller then checks the electrical charge storedby firing subsystem 76; as indicated at 906. This step ensures thatsufficient charge is present to melt the fusible member if the dangerouscondition is detected. Exemplary circuitry for detecting sufficientcharge is described in more detail below. If sufficient charge is notdetected within a determined time period, the logic controller respondswith an error signal 903.

In the sequence depicted in FIG. 3, after the predetermined checks arecompleted, logic controller 50 allows power to be sent to motor assembly16, as indicated at 907. It will be appreciated that the electricalsequence described above typically is completed within no more than afew seconds if no faults are detected. In addition to an initialpower-up sequence, logic controller 50 may be configured to perform anyof a variety of checks during operation. For example, the rotation ofthe blade may be monitored by known mechanisms and the firing system maybe disabled when the blade is not moving. This would allow the user totouch the blade when it is stopped without engaging brake mechanism 28.Various exemplary embodiments and implementations of a blade motiondetection system are described in U.S. Provisional Application Ser. No.60/225,094, entitled “Motion Detection System for Use in Safety Systemfor Power Equipment,” filed Aug. 14, 2000, by SD3, LLC.

It will appreciated that many variations on the logic sequence describedabove may be implemented within the scope of the invention. For example,some embodiments of logic controller 50 may include a battery, acapacitor or other charge storage device to ensure the detection andreaction subsystems will continue to function at least temporarily afterpower to the machine is turned off. As another example, power to themotor assembly may be shut off if an error occurs other than contactdetection such as incorrect blade-to-charge plate spacing, insufficientcharge on the charge storage devices, etc. Thus, logic controller 50 maybe implemented to provide any of a variety of safety and/or operationalfunctions as desired.

Additionally, since reaction subsystem 24 is configured to stop cuttingtool 14 upon contact with a user's body, it may also be desirable tostop motor assembly 16, or at least the portion of the motor assemblyadapted to drive the cutting tool, to prevent damage to the motor as ittries to drive the stalled cutting tool. However, since machine 10typically is designed with the expectation that the cutting tool maystop due to binding, etc., it will usually be sufficient to turn off themotor assembly within a few seconds. This can be accomplished simply bycutting power to the motor. For example, when machine 10 includes amagnetic contactor switch 48, the logic controller may be adapted tointerrupt the circuit holding the magnetic contactor closed so thatpower to the motor is interrupted. It should be understood that thisstep is optional, in that interrupting power to the machine's motorassembly is neither necessary nor sufficient to prevent serious injuryto the user when the user touches the machine's cutting tool. Therefore,the principal benefit of this step is to reduce the likelihood ofdamaging the motor assembly or drive system while the brake system ispreventing rotation or other movement of the cutting tool. It will beappreciated that there are many other suitable ways of stopping motorassembly 12 which are within the scope of the invention. As one example,power to the motor assembly may be controlled directly by safety stop 30(e.g., through solid state on/off switches, etc.). This embodiment isdescribed in more detail in U.S. Provisional Application Ser. No.60/225,200, entitled “Contact Detection System for Power Equipment,”filed Aug. 14, 2000, by SD3, LLC. Also, it is possible to simply allowexisting overload circuitry to trip in and turn off the stalled motor.

Since the contact detection subsystem described above relies on certainelectrical properties of the human body, the use of safety system 18while cutting some materials, such as foil-coated insulation, may causethe detection circuitry to falsely register contact with a user. Inaddition, as described in U.S. Provisional Application Ser. No.60/225,200, entitled “Contact Detection System for Power Equipment,”filed Aug. 14, 2000, by SD3, LLC, extremely green wood may cause falsetriggers in some types of detection subsystems due to the relativelyhigh dielectric constant of green wood. Therefore, it may be desirableto provide a manual bypass or override control that prevents the brakefrom operating for a particular cutting operation. A suitable overridecontrol may include a mechanical switch between fusible member 70 andfiring system 76. Alternatively, the switch may be a single-use switchconfigured to reset itself after each use. As a further alternative,safety system 18 may include sensors adjacent the workpiece to detectthe presence of foil, green wood, etc., and disable the reactionsubsystem automatically. This latter alternative relieves the user ofhaving to remember to disable and re-enable the brake system.

In any event, the override control may be configured in a variety ofways depending on the application and the level of safety desired. Forexample, the override control may be configured to time-out (i.e., turnoff) if the user does not switch the machine on within a predeterminedtime (e.g., 3, 5 or 10 seconds, etc.). This would prevent the user fromactuating the override control and then becoming distracted beforeproceeding to cut the workpiece and forgetting the safety system hadbeen disabled. In some embodiments, it may be desirable to allow a userto override the error caused by a failed self-test (e.g., no fusiblemember, insufficient stored charged, missing or incorrectly installedcartridge 80, etc.). In other embodiments, logic controller 50 may beconfigured to require that the detection and reaction subsystems areoperational before allowing the user to engage the override.

Typically, the override control is configured to reduce the likelihoodthat it will be actuated accidentally by the user. For example, theoverride control switch may be located away from the remaining operatorswitches and away from an area on machine 10 where the user is likely toaccidentally bump against while using the machine. Alternatively oradditionally, override control switch 48 may include a cover or similarbarrier which the user must remove or overcome before the switch can beactuated. Such covered switches are known to those of skill in the art.As an additional safety measure, logic controller 50 may be configuredto produce a visual and/or audible alarm or warning when the override isactuated. Furthermore, where logic controller 50 is adapted to controlthe supply of power to motor assembly 16, the logic controller may beconfigured to “pulse” the motor one or more times to alert the user thatthe blade is about to begin moving with the safety system disabled. Thiswould alert a user, who accidentally actuated the override while incontact with the blade, to quickly move away from the blade.

In view of the above considerations, an alternative embodiment of logiccontroller 50 may be configured to perform the self-test and detectionlogic shown schematically in FIGS. 4A-C. The main logic sequence,indicated generally at 910 in FIG. 4A, begins when machine 10 is firstconnected to power source 20, as indicated at 911. Logic controller 50begins sequence 910 by performing a system integrity check, as indicatedat 912. The system integrity check may include any one or more of avariety of checks which typically will vary depending on the particulartype and configuration of machine 10. In the exemplary embodiment,system integrity check 912 includes testing the sufficiency of powersource 20 (here, standard line current) by any suitable means which areknown to those of skill in the art. The system integrity check may alsoinclude driving the detection signal onto charge plate 44 and attemptingto detect the signal at charge plate 46. Failure to detect the detectionsignal at charge plate 46 may indicate a number of problems such as anelectronic failure in detection subsystem 22, a mis-positioned orgrounded charge plate, grounded blade, etc. Exemplary system integritycheck 912 also includes a pawl-to-blade spacing test to ensure that pawl60 is properly positioned adjacent blade 40 so that the pawl will engageand stop the blade if released. Exemplary mechanisms for detectingcorrect blade-to-pawl spacing are described in more detail below. If anyof the tests performed during system integrity check 912 is negative,logic controller 50 turns motor assembly 16 off (if on), as indicated at913, and outputs an error signal to the user, as indicated at 914. Oncethe user corrects the error and resets the logic controller (e.g., bydisconnecting and then reconnecting the power to machine 10), the systemintegrity check is repeated.

If system integrity check 912 is successful, logic controller 50proceeds to check fusible member 70 as well as the stored charge infiring subsystem 76, as indicated at 915. If either the fusible membertest or the stored charge test is negative, the logic controller turnsoff the motor assembly, indicated at 913, and then outputs an errorsignal, indicated at 914. It may be desirable to repeat step 915 one ormore times, or provide a delay between steps 912 and 915 to ensure thatfiring subsystem 76 has sufficient time to build up the electricalcharge.

If both the fusible member and firing subsystem tests are successful,the logic controller then proceeds to one of two operational loopsdepending on whether the user-operable override switch has beenactivated, as indicated at 916. It will be appreciated that testing fora user override signal after performing the fusible member/chargestorage test prevents a user from overriding safety system 18 unless thesafety system is functional. Thus, for example, if a contact detectionoccurs and the brake is triggered, the user cannot proceed to operatethe system until the fusible member, and/or pawl, and/or firingsubsystem, etc., is replaced (typically by replacing cartridge 80).Alternatively, step 915 may be eliminated from the main operationalloop. This would allow machine 10 to be operated regardless of whethersafety system 18 was completely functional by engaging the override.

In any event, if the override has been actuated, logic controller 50proceeds to operate in an override loop, as indicated at 917 anddetailed in FIG. 4B. Typically, logic controller 50 first outputs awarning signal, as indicated at 918 and described above. Next, at step919, the logic controller checks the status of START switch 48, which isoperable by a user to turn on motor assembly 16. As described above,logic controller 50 may be configured to read START switch 48 as being“on” only if it is actuated within a predetermined period after theoverride is enabled. If the START switch is “off,” logic controller 50turns off the motor assembly (if on), as indicated at 920, and exits theoverride loop as indicated at 921. As shown in FIG. 4A, the logiccontroller returns to the system integrity check at the end of theoverride loop. Thus, the logic controller will continue to perform thesystem integrity check and the fusible member/stored charge tests untilthe START switch is actuated. This ensures that if a user engages theoverride and then delays actuating the START switch, the system will notturn on the motor assembly if a failure occurs between the time theoverride is enabled and the time the START switch is actuated.

If, at step 919, the START switch is on, logic controller proceeds toturn on motor assembly 16, as indicated at 922. The motor assemblyremains on until STOP switch 48 is actuated by the user, as indicated at923. Once the STOP switch is actuated, logic controller 50 turns off themotor assembly, as indicated at 920, and exits the override loop at 921.As mentioned above, the logic controller returns to step 912 afterexiting the override loop.

If, at step 916, the override has not been engaged by the user, logiccontroller 50 proceeds to the detection loop 925, which is shown indetail in FIG. 4C. In the exemplary embodiment, detection loop 925 isdepicted with two logic paths which are executed simultaneously. In afirst path 926 the logic controller monitors detection subsystem 22,while in a second path 927 the logic controller continually rechecks thefusible member and stored charge in firing subsystem 76. This dual-pathoperation ensures that machine 10 will be shut down if a failure occurswhile the blade is in motion. It will be appreciated by those of skillin the art that the dual-path operation may be implemented in a varietyof ways including the use of interrupts, state machines, etc.Alternatively, the two paths may be implemented in a single sequentialloop. However, since testing of the stored charge consumes severalmilliseconds or even several seconds in some embodiments, it istypically desirable, in those embodiments, to execute both pathssimultaneously so that several milliseconds or more do not pass betweensuccessive contact detection measurements.

Path 927 includes testing fusible member 70 and the charge stored byfiring subsystem 76, as indicated at 928. This test is continuouslyrepeated unless and until either the fusible member test or the storedcharge test fails, at which point logic controller 50 turns the motorassembly off, as indicated at 929, and outputs an error message, asindicated at 930. The logic controller also stops executing test 928when it exits the detection loop or when an error in path 926 occurs, asdescribed below. The tests of fusible member 70 and firing subsystem 76at step 928 may be the same as, or different than, the tests that areused in the main loop at step 915. In any event, the logic controllermust be reset from step 930, as described above.

Path 926 is the contact detection path and includes testing forexcessive impedance loading on the blade, as indicated at 931. Step 931ensures that power will not be supplied to the motor assembly if thecapacitive load on the blade is so high that the detection subsystemmight not be able to detect a contact between the blade and the user.This might occur for a variety of reasons. For example, if the blade iscutting highly dielectric materials (e.g., green wood), the capacitiveload on the blade will increase. This issue is described in more detailin the incorporated references.

As another example, the user might accidentally actuate the START switchwhile in contact with the blade. Since some exemplary detectionsubsystems rely on a sudden change (rather than an absolute level) inthe signal detected at charge plate 46, step 931 ensures that the safetysystem will not allow the blade to begin rotating if the user istouching the blade when the START switch is actuated. In thisembodiment, the logic controller is configured to set the value forexcessive capacitive loading at approximately at least that amount ofloading caused when a user contacts the blade. However, it will beappreciated that logic controller 50 may be configured to recognize anydesired amount of capacitive loading as being excessive.

If the capacitive load on the blade is too high, logic controller 50outputs an error signal, at 932, and turns off motor assembly 16 (ifon), as indicated at step 933. The logic controller then exits thedetection loop, at 934, and returns to system integrity check 912 in themain operational loop shown in FIG. 4A. It will be appreciated thatsafety system 18 will not be enabled during the several seconds it takesthe blade to spin down. This is because the capacitive loading is toohigh to accurately detect contact with the user, and is likely totrigger even though no contact has occurred. In alternative embodiments,the logic controller may continue to monitor for contact detection whilethe blade is rotating and actuate the firing system if contact isdetected. Alternatively, the logic controller may be configured toactuate the firing system if the loading becomes too high.

Once the logic controller returns to the main loop after detecting ahigh capacitive loading error, the user may nevertheless operate machine10 by engaging the override. If the user does not actuate the override,safety system 18 will not supply power to motor assembly 16 until thecapacitive loading problem is corrected.

If, at step 931, the capacitive loading on the blade is within definedlimits, the logic controller proceeds to test the contact detectionsignal from detection subsystem 22, as indicated at 935. If contact isdetected, the logic controller determines whether the blade is rotating,as indicated at 936. If the blade is rotating, the logic controlleractuates the firing subsystem, at 937, turns off motor assembly 16, at929, and outputs an error, at 930. The logic controller must then bereset as described above.

However, if the blade is not rotating at step 936, then the logiccontroller outputs an error signal, at step 932, turns off the motorassembly (if on), at 933, and exits the detection loop, at 934. Thus, ifa user touches the blade when it is not rotating, the safety system willdetect the contact but will not actuate the firing subsystem. Thisallows a user to change or adjust the blade without actuating the brake.However, the user would typically remove power from machine 10 beforeadjusting or replacing the blade, in which case, neither safety system18 nor motor assembly 16 would be operable.

If no contact is detected at step 935, logic controller 50 checks thestatus of STOP switch 48, as indicated at 938. If the STOP switch isactuated, the logic controller turns off the motor assembly (if on), asindicated at 939, and checks for blade rotation, as indicated at 940. Ifthe blade is rotating, the logic controller loops back to step 931 sothat the contact detection is active as long as the blade continues torotate. Thus, if a user actuates the STOP switch and then contacts theblade before it spins down, safety system 18 will react to stop theblade. Once the blade ceases to rotate, the logic controller exits thedetection loop, as indicated at 934.

If the STOP switch has not been actuated at step 938, the logiccontroller checks the status of START switch 48, as indicated at 941. Ifthe START switch has been actuated, the logic controller turns the motorassembly on (if off), and loops back to repeat the contact detection, asindicated at 942. If the START switch has not been actuated, the logiccontroller turns off the motor assembly (if on), as indicated at 939,and checks for blade rotation, at 940. The logic controller continues toexecute the detection loop until the blade stops, at which point thelogic controller exits the detection loop, as indicated at 934. Thus,the logic controller is configured to continuously monitor for contactdetection whenever the blade is rotating and the user has not engagedthe override.

Those of skill in the art will appreciate that control subsystem 26 andlogic controller 50 may be implemented using many different componentsand many different configurations. Therefore, while two exemplaryimplementations are described below, it should be understood that anyother suitable implementation may be used.

A first exemplary implementation is illustrated schematically in FIG. 5.Logic controller 50 takes the form of a PIC16C63A-20/SO controlleravailable from Microchip Technology, Inc., of Chandler, Ariz. The logiccontroller is coupled to power source 20, contact detection subsystem22, and a user interface 178. The user interface may include anysuitable mechanism adapted to display signals to a user and to allow auser to input signals to the logic controller. Examples of suitable userinterface mechanisms which are known to those of skill in the artinclude lights, display screens, buzzers, sirens, switches, buttons,knobs, etc. In one exemplary embodiment depicted in FIG. 6, userinterface 178 includes START, STOP, and OVERRIDE switches to allow theuser to input control commands, and a pair of LED lights which indicatethe system status. The LED lights may indicate system status in avariety of ways such as color, blinking, etc.

The logic controller is also connected to control motor assembly 16 viaa suitable motor control circuit 174, such as is described in moredetail in U.S. Provisional Application Ser. No. 60/225,200, entitled“Contact Detection System for Power Equipment,” filed Aug. 14, 2000, bySD3, LLC, and to firing subsystem 76. When the logic controller receivesa signal from detection subsystem 22 that contact between the user andblade has occurred, the logic controller actuates firing subsystem 76and stops motor assembly 16. The operation and testing sequences areimplemented by software instructions stored within, and executable by,the logic controller. It will be appreciated that the softwareinstructions may take a variety of forms.

The logic controller of the exemplary implementation depicted in FIG. 5is configured to conduct a variety of self-tests before enabling powerto motor control 174, as well as whenever the blade is moving. Forexample, the logic controller is configured to evaluate the line voltagesupplied by power source 20, and to shut off the motor if the voltagedrops below a minimum value sufficient to operate the safety system. Thelogic controller is also adapted to test the contact sense signalreceived from the detection subsystem to ensure the charge plates arecorrectly positioned, that the detection signal is properly coupledacross the blade, and that the capacitive load on the blade is withindefined limits. Further, the logic controller is also coupled to a bladerotation sense component 177. Examples of suitable mechanisms fordetecting blade rotation are described in U.S. Provisional ApplicationSer. No. 60/225,094, entitled “Motion Detection System for Use in SafetySystem for Power Equipment,” filed Aug. 14, 2000, by SD3, LLC.

In addition, logic controller 50 is also adapted to detect whetherfiring subsystem 76 has sufficient stored charge to melt fusible member70. It will be appreciated that detection of sufficient stored charge inthe firing subsystem may be carried out in a variety of ways dependingon the configuration of the firing system. In each of the exemplaryimplementations described herein, firing subsystem 76 includes a single390 μF firing capacitor 620 configured to discharge through fusiblemember 70 via a suitable SCR 621 connected to ground. Exemplary firingsubsystems 76 are described in greater detail in U.S. ProvisionalApplication Ser. No. 60/225,056, entitled “Firing Subsystem for Use in aFast-Acting Safety System,” filed Aug. 14, 2000, by SD3, LLC.

In the implementation depicted in FIG. 5, the firing capacitor is bothcharged and tested by a buck-boost regulator 175, which is shown ingreater detail in FIG. 7. Buck-boost regulator 175 includes a buck-boostcharger 183 that steps up an 32-volt supply input to 180 volts forcharging the firing capacitor. Logic controller 50 provides a 125 khzinput to control the buck-boost cycle of the charger. A regulatorcircuit 184 monitors the voltage on the firing capacitor and turnscharger 183 on or off as necessary to maintain the charge near 180volts. Regulator circuit 184 is constructed with a predetermined amountof hysteresis so that the charger will go on when the firing circuitvoltage falls below 175 volts and turn off when the voltage reaches 180volts, as set by the voltage divider inputs and feedback to comparator185.

The output of comparator 185 is fed to logic controller 50. The logiccontroller monitors both the time required to charge and to dischargethe firing capacitor based on the state of the output of comparator 185.Thus, the controller can verify that the firing capacitor is operatingproperly and storing adequate charge. If the firing capacitor cannotreach 180 volts quickly enough or discharges too rapidly, the logiccontroller determines that the firing capacitor or charging system hasfailed and takes appropriate action based on its programming.

It should be noted that regulator circuit 184 measures the voltageacross the firing capacitor through fusible member 70. As a result, theregulator circuit is also testing the integrity of the fusible membersince a missing or failed fusible member would prevent the regulatorcircuit from detecting the voltage on the firing capacitor. Whiletesting both the firing capacitor charge and fusible member with asingle mechanism or test provides obvious savings of both processorcycle time and component costs, the fusible member may alternatively betested separately from the firing capacitor charge.

A second exemplary implementation of logic controller 50 is illustratedschematically in FIG. 8. Logic controller 50 is implemented by a 87C752controller available from Philips Semiconductor of Sunnyvale, Calif. Asin the first exemplary implementation described above, the logiccontroller of the second implementation is coupled to power source 20,contact detection subsystem 22, firing subsystem 76, user interface 178,motor control 174, and blade rotation sense 177. Suitable examples ofpower source 20, contact detection subsystem 22, and motor control 174are described in more detail in U.S. Provisional Application Ser. No.60/225,200, entitled “Contact Detection System for Power Equipment,”filed Aug. 14, 2000, by SD3, LLC. Exemplary firing subsystems 76 aredescribed in more detail in U.S. Provisional Application Ser. No.60/225,056, entitled “Firing Subsystem for Use in a Fast-Acting SafetySystem,” filed Aug. 14, 2000, by SD3, LLC. Exemplary circuitry andmechanisms for sensing blade rotations are described in more detail inU.S. Provisional Application Ser. No. 60/225,094, entitled “MotionDetection System for Use in Safety System for Power Equipment,” filedAug. 14, 2000, by SD3, LLC.

As shown in FIG. 9, the firing capacitor charging circuit for the secondimplementation is regulated by an enable line from logic controller 50.By deactivating the charging circuit, the logic controller can monitorthe capacitor voltage through an output to an analog-to-digitalconverter (A/D) line on the logic controller. When the capacitor is notbeing charged, it will normally discharge at a relatively known ratethrough the various paths to ground. By monitoring the discharge rate,the controller can insure that the capacitance of the capacitor issufficient to burn the fusible member. Optionally, the logic controllermay be configured to measure the voltage on the firing capacitor at aplurality of discharge intervals to evaluate the integrity of thecapacitor. In one embodiment, the logic controller measures thecapacitor voltage at three defined intervals during a discharge cycle,which should correspond to 3%, 5% and 7% of the full charge voltage. Thelogic controller may be configured to interpret a low voltage at any ofthe discharge intervals as a failure, or may require a low voltage attwo or more discharge intervals to indicate a failure.

As with the first exemplary implementation described above, the logiccontroller is configured to test the firing capacitor through fusiblemember 70, thereby simultaneously testing the fusible member.Alternatively or additionally, the logic controller may test the fusiblemember independently of the capacitor by monitoring the capacitorvoltage during charging.

As mentioned above, logic controller 50 may also be configured tomonitor the pawl-to-blade spacing. It is well known in the art that manycutting tools such as saw blades do not have precisely uniformdimensions. As a result, when a new blade is installed on a saw, thepawl may no longer be correctly spaced from the blade. An incorrectlypositioned pawl may slow the stopping speed of the pawl or prevent thepawl from stopping the blade. Therefore, to ensure the blade is stoppedwith uniform braking speed, it may be necessary to adjust the positionof the pawl whenever a blade is replaced. Exemplary mechanisms andmethods for automatically positioning the pawl are described in U.S.Provisional Application Ser. No. 60/225,212 entitled “Brake PositioningSystem,” filed Aug. 14, 2000, by SD3, LLC. However, regardless ofwhether the pawl is automatically positioned, configuring logiccontroller 50 to detect incorrect blade-to-pawl spacing provides anadditional level of assurance that a user is protected againstaccidental contact with the blade.

It will be appreciated that there are many ways in which incorrectspacing between blade 40 and pawl 60 may be detected. As one example,FIG. 10 illustrates a pawl 945 having a capacitive system for detectingcorrect pawl spacing. Similar to pawl 40 shown in FIG. 2, pawl 945 mayinclude a portion 946 that is beveled or otherwise shaped to quickly andcompletely engage the teeth of a cutting tool. In addition, pawl 945includes a pair of generally parallel, spaced-apart arms 947 whichextend beyond portion 946. Arms 947 are disposed to extend on eitherside of the blade, without touching the blade, when the pawl is in placeadjacent the blade. Each arm includes a capacitor plate 826 disposed onthe inside surface of the arm adjacent the blade. Conductive leads 949run from each capacitor plate 826 to suitable blade detector circuitry(not shown).

Capacitor plates 826 are positioned on arms 947 such that, when the pawlspacing is within a desired range, the blade extends between the twocapacitor plates. It will be appreciated that the capacitance acrossplates 826 will vary depending on whether the blade is positionedbetween the plates. The blade detector circuitry is configured to drivean electrical signal through conductive leads 949 and detect changes inthe capacitance across the plates.

Suitable circuitry that may be used with pawl 945 is well known to thoseof skill in the art. One exemplary pawl-to-blade spacing detectioncircuit is indicated generally at 824 in FIG. 11. As described above andin U.S. Provisional Application Ser. No. 60/225,200, entitled “ContactDetection System for Power Equipment,” filed Aug. 14, 2000, by SD3, LLC,and U.S. Provisional Application Ser. No. 60/225,211, entitled“Apparatus and Method for Detecting Dangerous Conditions in PowerEquipment,” filed Aug. 14, 2000, by SD3, LLC, one exemplary contactdetection system suitable for use with the present invention applies anelectrical signal to the blade via a drive plate (not shown). Thissignal can be picked up by either or both of plates 826 and monitored toinsure that it has an amplitude in a predetermined range. In particular,the amplitude detected by plates 826 will fall off rapidly with distancefrom the blade. Therefore, by monitoring the detected amplitude, properspacing can be verified. If the proper signal is not detected, circuit824 conveys an error signal to logic controller 50, which preventsoperation of machine 10 until proper pawl-to-blade spacing is detected.Other examples include circuits similar to the exemplary contactdetection circuits described in U.S. Provisional Application Ser. No.60/225,200, entitled “Contact Detection System for Power Equipment,”filed Aug. 14, 2000, by SD3, LLC.

Capacitor plates 826 can optionally be shaped to detect when the pawl istoo close to the blade as well as not close enough. Alternatively, twopairs of capacitor plates may be positioned on the pawl: one pair todetect if the pawl is too close to the blade, and the other pair todetect if the pawl is too far from the blade. In any event, the detectorcircuitry is configured to transmit an error signal to logic controller50, which then takes appropriate action.

While one exemplary automatic pawl spacing detection system has beendescribed above, it will be appreciated that there are many possiblevariations within the scope of the invention. For example, bothcapacitor plates may be positioned on the same side of the blade ratherthan on opposite sides. The capacitor plates and/or blade detectioncircuitry may be separate from the pawl. In the latter case, forexample, the capacitor plates and detection circuitry may be mounted ona separate electronics board associated with the pawl. Alternatively,the capacitor plates may be replaced with one or more light-emittingdiodes and detectors such that, when the pawl is properly positioned,the blade obstructs the optical path between the diodes and detectors.Other methods of detecting the proximity of the blade to the pawl arealso possible. As a further option, capacitor plates 826 may function ascharge plates 44, 46 as well as pawl-spacing detectors. In addition, adetection plate may be mounted on beveled face 946 of the pawl. Thisplate can be used to detect the drive input signal used for contactdetection. The amplitude of the signal detected at the plate will beinversely proportional to the space between the plate and the teeth ofthe blade. If this signal does not have an amplitude over a giventhreshold, the system would interpret this as indicating that the pawlface is not close enough to the blade.

In embodiments where portions of safety system 18 are mounted in areplaceable cartridge 80, logic controller 50 may also be configured todetect whether the cartridge is properly connected to the remainder ofthe safety system. One exemplary method of testing for an operableconnection with the cartridge is by testing a component mounted in thecartridge (e.g., the fusible link, charge stored by firing system,etc.). Alternatively, a cable (not shown) connecting cartridge 80 tologic controller 50 may include a separate signal line which is groundedor otherwise biased when the cartridge is connected. In addition todetecting an operable connection to the cartridge, the correctblade-to-pawl spacing may be detected by measuring theblade-to-cartridge spacing. For example, capacitor plates 826 may beplaced on cartridge housing 82 rather than on the pawl itself.Furthermore, failure of the blade-to-cartridge spacing test could alsobe used to detect an inoperable connection to the cartridge.

As described above, the present invention provides a reliable, effectiveand fast-acting system for preventing serious injuries to operators ofpower cutting machinery. While a few specific embodiments of safetysystem 18 and particularly control subsystem 26 have been described,those of skill in the art will appreciate that the present invention maybe adapted in numerous ways for use in a wide variety of applications.Therefore, it will be understood that all such adaptations andapplications are within the scope of the invention.

It is believed that the disclosure set forth above encompasses multipledistinct inventions with independent utility. While each of theseinventions has been disclosed in its preferred form, the specificembodiments thereof as disclosed and illustrated herein are not to beconsidered in a limiting sense as numerous variations are possible. Thesubject matter of the inventions includes all novel and non-obviouscombinations and subcombinations of the various elements, features,functions and/or properties disclosed herein. No single feature,function, element or property of the disclosed embodiments is essentialto all of the disclosed inventions. Similarly, where the claims recite“a” or “a first” element or the equivalent thereof, such claims shouldbe understood to include incorporation of one or more such elements,neither requiring nor excluding two or more such elements.

It is believed that the following claims particularly point out certaincombinations and subcombinations that are directed to one of thedisclosed inventions and are novel and non-obvious. Inventions embodiedin other combinations and subcombinations of features, functions,elements and/or properties may be claimed through amendment of thepresent claims or presentation of new claims in this or a relatedapplication. Such amended or new claims, whether they are directed to adifferent invention or directed to the same invention, whetherdifferent, broader, narrower or equal in scope to the original claims,are also regarded as included within the subject matter of theinventions of the present disclosure.

1. A woodworking machine comprising: a cutting tool for cuttingworkpieces; at least one motor configured to drive the cutting tool; adetection system configured to detect a dangerous condition associatedwith the cutting tool; a brake mechanism having at least one replaceablesingle-use component, and controllable to stop the cutting tool if thedangerous condition is detected; and a control system configured todetect whether the single-use component has been used, and to disablethe at least one motor until the single-use component has been replaced.2. The machine of claim 1, where the at least one replaceable single-usecomponent includes a fusible member.
 3. The machine of claim 2, wherethe control system is configured to detect whether the fusible memberhas been melted.
 4. The machine of claim 2, where the control system isconfigured to test the electrical continuity of the fusible member. 5.The machine of claim 2, where the brake mechanism includes a capacitoradapted to store electrical charge, and where the control system isconfigured to determine the capacitance of the capacitor.
 6. The machineof claim 5, where the control system is configured to measure thecapacitance of the capacitor through the fusible member.
 7. The machineof claim 2, where the brake mechanism includes a capacitor adapted tostore electrical charge, and where the control system is configured todetermine the electrical charge stored on the capacitor.
 8. The machineof claim 7, where the control system is configured to measure theelectrical charge stored on the capacitor through the fusible member. 9.A method of controlling a saw, where the saw includes a blade driven bya motor, a detection system adapted to detect a dangerous conditionbetween a person and the blade, and a reaction system associated withthe detection system to cause a predetermined action to take place upondetection of the dangerous condition, the method comprising: checking tosee whether the detection system is functioning; checking to see whetherthe reaction system is functioning; and powering the motor to drive theblade if the detection and reaction systems are functioning.
 10. Themethod of claim 9, further comprising checking to see whether thedetection system is functioning while the motor is running, and shuttingthe motor off if the detection system is not functioning while the motoris running.
 11. The method of claim 9, further comprising checking tosee whether the reaction system is functioning while the motor isrunning, and shutting the motor off if the reaction system is notfunctioning while the motor is running.
 12. A woodworking machinecomprising: a cutting tool for cutting workpieces; at least one motorconfigured to drive the cutting tool; a detection system configured todetect a dangerous condition between a person and the cutting tool; areaction system controllable to disable the cutting tool if thedangerous condition is detected; and a control system configured todetermine the operability of the detection system and to disable themotor if the detection system is inoperable.
 13. The machine of claim12, where the detection system imparts an electrical signal to thecutting tool and monitors the electrical signal for changes indicativeof the dangerous condition.
 14. The machine of claim 12, where thecontrol system is adapted to determine the operability of the detectionsystem without having to operate the reaction system.
 15. The machine ofclaim 12, where the motor is controllable by the control system, wherethe control system is configured to determine the operability of thedetection system prior to actuation of the motor, and where the controlsystem is further configured not to actuate the motor unless thedetection system is operational.
 16. The machine of claim 12, where thedetection system includes at least one charge plate, where the motor iscontrollable by the control system, where the control system isconfigured to detect a predetermined signal on the charge plate prior toactuation of the motor, and where the control system is furtherconfigured not to actuate the motor unless the predetermined signal isdetected.
 17. A woodworking machine comprising: a support structure; acutting tool adapted to move to cut a workpiece, where the cutting toolis supported by the support structure; a motor adapted to drive thecutting tool; a detection system adapted to detect a dangerous conditionbetween the cutting tool and a person; a reaction system adapted toperform a specified action upon detection of the dangerous condition;and a self-test system adapted to test the operability of at least aportion of the detection system and to disable the motor if the testedportion of the detection system is inoperable.
 18. The machine of claim17, where the detection system imparts an electrical signal to thecutting tool and monitors the electrical signal for changes indicativeof the dangerous condition.
 19. The machine of claim 17, where theself-test system is configured to determine the operability of thedetection system prior to actuation of the motor, and where the motor isnot actuated unless the detection system is operational.
 20. The machineof claim 17, where the detection system includes at least one chargeplate, where the self-test system is configured to detect apredetermined signal on the charge plate, and where the motor is notactuated unless the predetermined signal is detected.